1. Field of the Invention
The invention relates to an electronically controlled throttle apparatus for controlling intake air volume of an internal-combustion engine.
2. Description of the Related Art
A conventional electronic throttle control apparatus includes a throttle body forming an intake passage, a throttle valve for opening and closing the intake passage, a motor for driving the throttle valve, and a throttle sensor for detecting an actual opening angle of the throttle valve. Rotation of the motor is transmitted to the throttle valve by way of a reduction mechanism, driving (opening or closing) the throttle valve, and the actual opening angle of the throttle valve detected by the throttle sensor is so controlled as to be a target opening angle.
This kind of electronic throttle control apparatus, for example, learns a fully closed position of the throttle valve as a reference position, and controls the opening angle of the throttle valve, based on the learned fully closed position.
When the fully closed position is to be learned, abutting against the fully closing stopper is determined. To control the opening angle of throttle valve with a high degree of precision, therefore, a high-precision determination of abutting against the fully closing stopper is required. In the abutting determination, it is detected that the throttle valve has hit the fully closing stopper (the fully closed position) when a duty ratio has become larger than a predetermined threshold value. (JP2005-171915A)
The electronic throttle control apparatus is arranged to set a control opening lower limit which is larger than a control reference opening angle by a predetermined opening angle (for example, about 0.5 deg.), so that the opening angle of the throttle valve may not become smaller than this control opening lower limit.
Learning of the fully closed position is determined depending on the condition of an accelerator position or a battery voltage before an engine starts. For some reason, however, the fully closed position may not be learned before the engine starts. Accordingly, a new electronic throttle control apparatus has been proposed for learning the fully closed position not only before but also after start of the engine. In this electronic throttle control apparatus, when the throttle valve abuts against the fully closing stopper, this position is adopted to be learned and updated as the actual fully closed position (JP 7-269406(1995)A).
In the conventional throttle control apparatus, however, abutting of the throttle valve against the fully closing stopper may not be detected accurately. For example, when the material of gear units and others for composing a throttle system is a resin or the like, abutting against the fully closing stopper may not be detected accurately. That is, when the gear unit or the like is made of resin or the like, the amount of distortion or the dimension of components of the gear unit may largely change depending on the ambient temperature when the throttle valve abuts against the fully closing stopper. Thus, even if a duty ratio is smaller than a predetermined threshold value, the throttle valve may sometimes abut against the fully closing stopper.
As a recent trend for improving fuel economy or efficiency or others, the idling speed is set to a lower level. Further, there is an increasing demand for controlling the opening angle of the throttle valve until the throttle valve abuts against the fully closing stopper. Such circumstances increase the need for detecting the abutting position of the throttle valve against the fully closing stopper more accurately. This is because if abutting against the fully closing stopper is not detected accurately, the motor is driven continuously in order to close the throttle valve even though the throttle valve actually abuts against the fully closing stopper, resulting in an overloaded motor. Accordingly, the motor performance may drop or the motor may be broken down.
In a conventional electronic throttle control apparatus, moreover, an idling speed may not be lowered to a target speed or may be increased too high, so that a desired idling speed may not be maintained. The reason is as follows. Due to problems in an assembling precision of the throttle control apparatus or temperature characteristics of the throttle sensor, a control reference opening angle (a learned fully closed angle) and an actual (mechanical) fully closed angle may not coincide perfectly (that is, an error may occur).
Specifically, if the control reference opening angle becomes larger than the actual fully closed angle, when a target opening angle smaller than the control opening lower limit is calculated, the idling speed may not be lowered to the target rotating speed. If the control reference opening angle is smaller than the actual fully closed angle, when a target opening angle smaller than the actual fully opening angle is calculated, the throttle valve abuts against the fully closing stopper. When this abutting is detected, the control reference opening angle is changed to the actual throttle valve opening angle during the abutting determination. However, the control reference opening angle is correspondingly increased. Accordingly, the target opening angle calculated based on the control reference opening angle also becomes large, thereby increasing the idling speed.